This invention relates generally to apparatus and a form for making an air formed fibrous web and to an absorbent core formed by such a web. The absorbent core can be used for applications such as disposable diapers, child's training pants, feminine care articles, incontinence articles, and the like.
In the general practice of forming fibrous web materials, such as air formed fibrous webs, it has been common to use a fibrous sheet of cellulosic or other suitable absorbent material which has been fiberized in a conventional fiberizer, or other shredding or comminuting device, to form discrete fibers. In addition, particles of superabsorbent material have been mixed with the fibers. The fibers and superabsorbent particles have then been entrained in an air stream and directed to a foraminous forming surface upon which the fibers and superabsorbent particles have been deposited to form an absorbent fibrous web. An absorbent core formed in this fashion has a liquid holding formation which is intended to be the primary repository for liquid to be held by the absorbent core. Thus, the liquid holding formation has conventionally been formed to have a greater amount of fibrous and superabsorbent material (SAM) than surrounding regions and is generally thicker than the surrounding regions of fibrous material.
The forming surfaces used in such systems have been constructed with a perforated plate or wire screen grid and can typically employ a pneumatic flow mechanism, such as vacuum suction apparatus, to produce a pressure differential across the forming surface. The pressure difference causes an airflow through the openings or perforations in the plate or screen of the forming surface. The use of vacuum suction to draw the air-entrained fiber stream onto the forming surface, and pass the airflow through the forming surface is presently employed in high-speed commercial operations. In operation, fibrous material is deposited on the forming surface as it passes through a chamber of the fluent fibers, forming a layer of fibrous material on the forming surface. As the forming layer increases in basis weight (i.e., weight of deposited fiber and SAM per unit area) resistance to air flow through the layer increases. One problem which arises as a result of increased resistance is inadequate deposition of fiber in the area which forms the liquid holding formation. Stated another way, the liquid holding formation may not have a sufficiently higher basis weight than the surrounding regions. The problem can be particularly acute when the area in which material is to be deposited is narrow. There are also problems associated with increased flow resistance such as high weight variability in the absorbent core, fiber damaged caused by excessive retention in the fiberizer, and poor strength caused by insufficient entanglement of fibers in the absorbent core.
The liquid holding formation is typically formed through the provision of a pocket in the forming surface. It has been found that instead of depositing fibrous material to a greater depth in the pocket the depth of material often is nearly the same as in the shallower surrounding regions. Typically, the absorbent core is cut or scarfed after forming on the forming surface so that the surrounding areas end up with a lesser thickness than the region of the liquid holding formation. However, there is often a marked dip in the scarfed surface of the finished absorbent core in the liquid holding formation area indicating that less than a full desired thickness of fibrous material has been deposited. The reduction in the amount of fibrous material (and superabsorbent material) corresponds to a reduction in the quantity of liquid which can be held by the liquid holding formation and the absorbent core. Attempts to remedy this and other problems associated with air flow by control of the vacuum pressure on the forming surface have been complicated and difficult to control.